Effect of Mn loading onto hydroxyapatite supported Mn catalysts for toluene removal: Contribution of PCA assisted ToF-SIMS

Abstract : A series of hydroxyapatite (Hap; of ideal formula: Ca10(PO4)6(OH)2) supported MnOx materials (MnxHap; x = Mn wt.%:2.5, 5.0, 10, 20, and 30 based on MnO2) has been tested prepared by wet impregnation using manganese (II) nitrate tetrahydrate as metal precursor followed by calcination at 400 °C in toluene total oxidation (800 ppmv in dry air) at 220 °C for 40 h. It was shown that the fresh catalysts can be ranked as follow by decreasing activity: Mn5Hap > Mn2.5Hap > Mn10Hap > Mn20Hap > Mn30Hap and that Mn10Hap exhibited the best resistance to deactivation. Investigation of the surface state of the fresh and used MnxHap catalysts has been performed using X-ray photoelectron spectroscopy (XPS) and principal component analysis (PCA) assisted static time-of-flight secondary-ion mass spectrometry (ToF-SIMS). Based on the XPS results a Mn speciation scheme can be proposed: initial incorporation of Mn2+ into Hap (2.5 wt% Mn) followed by additional well dispersed oxidized manganese species on Hap (5–10 wt% Mn) to end up with the formation Mn2O3/ε-MnO2 particles detected by XRD (20–30 wt% Mn). Principal components analysis (PCA) has been applied to the (+) ToF-SIMS spectra taking into account of all the detected significant manganese containing secondary ions. PC1 (75% of the variance) discriminates well the fresh and used samples in a similar way and was related to the surface manganese content. The resulting volcano plot obtained by plotting PC2 (12% of the variance) as a function of the resistance to deactivation discriminates the catalysts in the same way as depicted in the PC2 score plot. Consequently also taking into account of the PC2 loading plot it was suggested that a redispersion of Mnn+/Ca2+ takes place with time on stream preferentially for MnxHap (x = 5–10 wt%) improving their catalyst stability.